1. Field of the Invention
The present invention relates to a mass spectrometry method and a mass spectrometry apparatus for d halide compound, and more particularly relates to a mass spectrometry method and a mass spectrometry apparatus for a halide compound using soft ionization utilizing ion attachment in ionizing the halide compound for the purpose of mass spectrometry.
2. Description of the Related Art CF4, C4F6, and other perfluoro compounds (PFC), CH3 and other hydrofluorocarbons (HFC), and SF3, NF3 and other gases are utilized in various industries. These fluoride compounds are extremely stable, but have an extremely large effect on global warming. There have therefore been calls for reducing the amount of their emission on a global scale. In particular, in semiconductor and electronic component manufacturing facilities, gases are inevitably being discomposed and emitted Therefore, emphasis is currently being placed on precisely measuring what types of ingredients of the gases are being emitted in exactly what amounts.
In the past, the general practice had been to use mass spectrometry for measuring the ingredients and amounts of gas. Mass spectrometry measures the mass of the gas (or molecular weight) to identify the ingredients of the gas and measure the amount of the gas. In mass spectrometry, a positive charge or negative charge is given to the neutral gas molecules to Ionize them, then the gas molecules are led into a specific electrical field or magnetic field space and the electrical force or magnetic force applied to the ions is changed to obtain a mass spectrum with mass as its abscissa and the amount of tons as its ordinate. For this mass spectrometry, there have conventionally been several methods in accordance with the means of ionization used, that is, (1) the method of utilizing electron impact, (2) the method of utilizing electron attachment, and (3) the method of utilizing cation (positive ion) attachment. These mass spectrometry methods will be explained in brief below and their problems are pointed out.
(1) Electron Impact Mass Spectrometry:
In a mass spectrometry method utilizing electron impact (EI), electrons having an energy of, for example, 50 to 100 eV or so are made to collide with gas molecules to strip electrons from the gas molecules and convert the gas molecules to positive ions. This Is the most generally used method because the hardware is simple.
(2) Electron Attachment Mass Spectrometry;
This mass spectrometry method has been developed in recent years and uses the action of electron attachment (EA). That is, electrons having a low energy of not more than 10 eV are made to attach to the gas molecules to give the gas molecules a negative charge as a whole and ionize them. This method has the advantage of involving less excess energy and resulting in less dissociation compared with electron impact. The method was developed with the intent of precise mass spectrometry of fluoride compounds. As a technical reference of the related art, Oyo Butsuri (Applied Physics). vol. 68. no. 10, p. 1148 (1999) and Review of Scientific Instruments, vol. 69, no. 1., p. 116 (1998) may be mentioned.
(3) Cation Attachment Mass Spectrometry
This mass spectrometry method causes cations (or positive ions) to be attached to the gas molecules for ionization Instead of the afore-mentioned electron attachment. This method has been in existence for a comparatively long time, therefore has been mainly used in the field of organic spectrometry. The method alms at precise mass spectrometry of gas molecules without causing dissociation. The method of using positive charge metal ions of an alkali metal is effective. In practice, the efficacy has been confirmed for hydrocarbons with small electron affinity. Up until now, various systems have been proposed by Hodge, Bombick, Fujii, etc. These systems will be explained in brief next.
The Hodge system is described in Analytical Chemistry, vol. 48, no. 6. p. 825 (1976). The system proposed by Hodge utilizes Li+ as the alkali metal ions. Li+ is generated by heating an emitter including an Li oxide. The Li+ generated from the emitter moves to the flight region, then is guided to the reaction chamber containing the gas molecules to be detected. Therefore, the emitter is placed in a region outside the reaction chamber. The ionization is performed in the reaction chamber. The gas molecules with the Li+ attached as a result of the ionization are withdrawn from the reaction chamber and transported to a quadrapole mass spectrometer for mass spectrometry. This system is an indirect attachment method which causes Li+ to be attached to the gas molecules being detected. It is said that direct attachment is difficult with this system. In indirect attachment. the Li+ is first attached to a reaction gas, then the Li+ is moved from the reaction gas, with the Li+ to the gas being detected. In direct attachment. the Li+ is attached directly to the gas molecules being detected without interposition of a reaction gas. The above reference gives working data to explain the indirect attachment in the above way. The reasons given are that there is little difference in the sensitivity by the detected gas (ease of attachment of cations) and there Is little dissociation. As opposed to this, it indicates that when using direct attachment, dissociation occurs when the detected gas is a fluoride compound etc. so, direct attachment is difficult. In the above configuration, a large amount of reaction gas (isobutane or other hydrocarbon) is introduced into the reaction chamber.
The system of Bombick is described in Analytical Chemistry, vol. 56, no. 3, p. 396 (1984). The system proposed by Bombick utilizes K+ as the alkali metal ions. An emitter containing potassium (K) oxide is placed in the reaction chamber. Only the gas being detected is introduced into the reaction chamber. No other reaction gas is introduced. Therefore, the K+ released from the emitter in the reaction chamber directly attaches to the molecules of the detected gas. That is, the system of Bombick is a direct attachment system.
Problems in Electron Impact Mass Spectrometry:
In this method, the electron impact is a physical action, so a high energy is given to the detected gas. Therefore, if electron impact is used for gas molecules of a fluoride compound or other halide compound, since the binding energy of atoms in the gas molecules is usually small, the excess energy of the electron impact will cause the gas molecules to dissociate into fragments along with the ionization. Therefore, the peak due to the gas molecules not dissociating in the mass spectrum is called the molecule peak (or parent peak), while the peaks of the dissociated fragments are called the fragment peaks. With molecules with large number of atoms such as the above halide compounds, however, fragment peaks appear due to dissociation and the inherent molecule peak cannot be discerned well at all. FIG. 4 shows an example of a mass spectrum obtained by ionization of molecules of a fluoride compound, that is. C4F8, by electron impact by an electron energy of 70 eV. In FIG. 4, the abscissa indicates the mass, while the ordinate indicates the amount of ions. The mass of the C4F8 molecule is 200 amu (atomic molecular units), but the molecule peak does not appear much at all in the mass spectrum. Only the fragment peaks of C3F5, C2F4, CF, etc. appear. In the past, the practice had been to deduce the inherent molecule peak, that is, the mass and amount of the molecules, based on the state of appearance of these fragment peaks. In this way, precise mass spectrometry of a halide compound was difficult with mass spectrometry using electron impact.
Problems in Electron Attachment Mass Spectrometry:
This method has currently reached a level of measurement sensitivity practical for halide compounds. Compared with the method of utilizing electron impact, there is Indeed less dissociation. Even so, however, it is not possible to sufficiently suppress dissociation. Even with electron attachment mass spectrometry, precise mass spectrometry of a halide compound is difficult.
Problems in Cation Attachment Spectrometry
This method, as explained above, includes various systems such as the Hodge system, Bombick system, and Fujii system. The Hodge system is an indirect attachment method as shown in the working data. In the case of the Hodge system, direct attachment does not allow precise measurement of the molecule peak of the gas molecules being detected, because, if using a fluoride compound, dissociation occurs along with the Li+ attachment and the Li+ only attaches to the fragments after dissociation. Further, in the Bombick system, when the potential of the emitter becomes higher (more than 5V), dissociation occurs due to the reaction of the detected gas. Further, the concentration of the detection gas becomes high, so the problem arises of the molecules of the detected gas easily bonding together and forming clusters. In addition, there is the problem of emitter damage, that is, the problem of the emitter suffering large damage due to the active fragments dissociated at the surface of the emitter. Among the above systems, the Fujii system has the advantage in structure of eliminating the above problems relating to cation attachment mass spectrometry and enables precise measurement of hydrocarbons and their radicals and other unstable molecules. Further, the Fujii system is being improved and applied to inventions for ion attachment in atmospheric reaction chambers (Japanese Examined Patent Publication (Kokoku) No 7-48371), inventions for attaching Ions to neutral active species (Japanese Unexamined Patent Publication (Kokai) No. 6-11485), etc. As explained above, however, the cation attachment method of the Fujii system is not currently being applied to mass spectrometry for measuring the amount of emission of halide compounds, Including fluoride compounds, having a large impact on global warming.
An object of the present invention is to provide a mass spectrometry method and mass spectrometry apparatus for a halide compound which apply cation attachment of the Fujii system to mass spectrometry of a halide compound and enable precise measurement of fluoride compounds etc. having a large impact on global warming.
The mass spectrometry method and mass spectrometry apparatus of a halide compound according to the present Invention are configured as follows to achieve the above object. Below, first, the principle of the present invention will be explained, then the means of the invention and action of the same will be explained.
Principle of Invention: p The fact that electrons easily attach to fluoride compounds (large electron affinity) has been well known in the past. Therefore, electron attachment mass spectrometry had been applied to actual measurement of fluoride compounds. At the same time, the general belief had been the vague xe2x80x9cpositive charges or cations do not attach easily to fluoride compounds to which negative charges or electrons easily attachxe2x80x9d. This had been the conventional belief judging from the fact that there is overwhelmingly less F+ compared with the negative ions F. Therefore, most persons involved in the mass spectrometry method had not even imagined applying the cation attachment mass spectrometry method to fluoride compounds.
The present Inventor, however, engaged in a careful study and as a result predicted that cation attachment mass spectrometry could be applied to fluoride compounds. A precise explanation of the applicability should be made from the results of theoretical calculations by a supercomputer. Very simply, however, it can be said that, since the distribution of electrons of the compound is lopsided, positive charges easily attach to the side opposite to where the negative charges easily attach. That is, the ease of attachment of the cations is determined by the lopsided distribution of electrons and is not inversely proportional to the ease of attachment of the electrons. Therefore, It is, predicted that cations will sufficiently attach to even fluoride compounds to which electrons easily attach.
Further, as clear from the above Hodge reference, it has been suggested that, basically, cation attachment mass spectrometry can be applied to a fluoride compound. In this case, however, as explained above, it had been understood that application was difficult because dissociation would occur when direct attachment was applied to a fluoride compound, and that indirect attachment using a reaction gas would be more practicable.
As opposed to this, the present inventor did not believe that dissociation of the fluoride compound in the direct attachment method was in principle an unavoidable phenomenon. The reasons were as follows.
In the Hodge system, the pressure of the region of placement of the emitter is a low 103 Pa, so the Li+ produced from the emitter flies without absorption of energy by the atmospheric gas (reduction of translational energy). Further, the pressure of the reaction chamber is a somewhat high 1.3 Pa, but the mean free path at that pressure is about 5 mm and almost no absorption of energy by collisions can be expected. Therefore, the Li+ collides with the detected gas at a high energy of 1 to 2 eV and dissociation easily occurs. Incidentally, due to the high energy impact, the probability of not again separating after impact (attachment efficiency) is also considerably degraded.
Further, excess energy is produced in the molecules after attachment of Li+. Due to the attachment of Li+, the molecules as a whole stabilize in terms of energy. That is, the internal energy becomes low. The difference In internal energy before and after attachment is the excess energy. This causes dissociation. In particular, this excess energy is large In direct attachment. Further, in the Hodge system, the pressure of the reaction chamber is an insufficiently high 1.3 Pa, so excess energy remains in the molecules without being dispersed and absorbed by the atmospheric gas and causes dissociation.
That is, the present inventor judged that dissociation occurs in fluoride compounds etc. by the direct attachment of the Hodge system since the pressure in the region of placement of the emitter and the reaction chamber is not sufficiently high. Therefore, the present inventor judged that by making the pressure of the region of placement of the emitter and the reaction chamber sufficiently high, the translational energy of the Li+ in the region of placement of the emitter can be reduced and the excess energy of the Li+ attached detected gas can be dispersed and accordingly fluoride compounds can be measured without dissociation. Making the pressure of the region of placement of the emitter and the reaction chamber sufficiently high was predicted to be substantially Identical to the cation attachment mass spectrometry of the Fujii system.
Therefore, the present inventor conducted the following experiment to confirm the prediction that the Fujii system cation attachment mass spectrometry can be applied to a fluoride compound. The detected gas was made the representative PFC (perfluoro compound) of C4Fn, the cations were made Li+ , the pressure of the reaction chamber was made 133 Pa, and the inert gas was made N2. The hardware configuration will be explained in the second embodiment. As a result of the measurement test, as shown in the mass spectrum of FIG. 3, peaks only appeared at the 207 amu of C4FnLi+, 35 amu of N2Li+ corresponding to the molecule peak of N2, 25 amu of H2OLi+ corresponding to the molecule peak of H2O included In the gas as an impurity, and 7 amu of unattached Li+. No fragment peaks at all appeared. That is, it was confirmed that the mass of the gas molecules of the detected gas was precisely measured without the occurrence of dissociation even when using cation attachment mass spectrometry for a fluoride compound. Note that 133 amu is the Cs impurity included In the emitter itself. Note further that making the pressure of the region of placement of the emitter and the reaction chamber higher is a preferable condition, but is not an essential condition when considering future advances. For example, if electrically giving conditions assisting the reaction, it may be possible to perform the mass spectrometry by cation attachment to a fluoride compound under relatively low pressure conditions.
Configuration of Invention:
A mass spectrometry method for a halide compound according to the present invention is configured as follows.
The mass spectrometry method is a method comprising ionizing a detected gas and then utilizing electromagnetic force to measure the mass of the molecules of the detected gas. The detected gas is a halide compound. Positive charge metal ions are attached directly on the halide compound to ionize it. According to this mass spectrometry method, the technique for mass spectrometry of a fluorine compound or other halide compound comprises directly and softly attaching positive charge metal ions on the halide compound, then transporting the molecules of the halide compound with the metal ions to a quadrapole mass spectrometer to measure the same, so it is possible to directly observe the molecule peaks in the mass spectrum without causing dissociation.
In the above mass spectrometry method, preferably the main component in the atmosphere of the reaction region for the ionization is a gas to which metal ions attach less easily than to the halide compound. By making a gas to which the metal Ions attach less easily the main component in the reaction region, metal ions attach directly more easily on the halide compound of the sample gas.
In the mass spectrometry method, preferably the main component is a gas selected from any of He, Ar, Ne, H2, and N2, As the gas to which the metal ions attach less easily, a stable inert gas is used.
In the above mass spectrometry methods, preferably a pressure of an atmosphere of at least the reaction region is maintained at a predetermined high pressure by the gas forming the main component. To set the pressure of the atmosphere of the reaction region high, as explained in the principle of the invention as well, is a preferable condition from the viewpoint of the prevention of dissociation when attaching ions on a halide compound by direct attachment. As explained earlier, however, there is a possibility that the pressure can be lowered as well, so this cannot be said to necessarily be a necessary condition.
In the above mass spectrometry methods, preferably the pressure of the atmosphere of the reaction region is at least 10 Pa, more preferably at least 100 Pa.
In the above mass spectrometry methods, preferably the pressure of the atmosphere of a region wherein the metal ions fly until entering the reaction region Is at least 10 Pa. In making the pressure of the atmosphere of the reaction region high, the pressure of the atmosphere of the metal ion flight region between the emitter emitting the metal ions and the reaction region is preferably also made higher.
In these mass spectrometry methods, preferably the halide compound includes a carbon atom and a fluorine atom.
In these mass spectrometry methods, preferably the metal ion is any of Li, K, Na, Rb, Cs, Al, Ga, and In.
Further, the mass spectrometry apparatus for a halide compound according to the present invention is configured as follows.
This mass spectrometry apparatus is provided with an emitter for emitting metal ions, a reaction chamber where the detected gas Is Introduced and ionized by the metal ions, an electromagnetic guide (aperture, condensing lens, etc.) for guiding molecules of the ionized detected gas, and a mass spectrometer for measuring the molecules guided by the guide (quadrapole mass spectrometer etc.) and causes the metal ions emitted from the emitter to fly to the reaction chamber to ionize the detected gas. Further, the detected gas is a halide compound and further provision is made of a first gas source (sample gas source) for feeding a halide compound to the reaction chamber and a second gas source for feeding to the reaction chamber a gas to which the metal ions attach less easily than to the halide compound (N2 gas source).
According to the apparatus of the above configuration, it is possible to work the mass spectrometry method of a halide compound according to the present invention and possible to apply the cation attachment method to mass spectrometry of a halide compound and thereby accurately measure a fluoride compound etc.
In the mass spectrometry apparatus, preferably the gas to which the metal Ions attach less easily is any of He, Ar, Ne, H2, and N2. As the gas to which the metal ions attach less easily, an inert gas having stable properties Is preferred.
In the mass spectrometry apparatus, preferably further provision is made of a pressure adjusting means for adjusting an internal pressure of the reaction chamber to a predetermined high pressure and the internal pressure of the reaction chamber is held at the predetermined high pressure by the pressure adjusting means. By setting such pressure conditions, it becomes easy to make metal ions directly attach on the molecules of the halide compound, dissociation etc. is prevented, and a practical apparatus is obtained.
In the mass spectrometry apparatus, preferably the internal pressure of the reaction chamber is at least 10 Pa. As the above high pressure, at least 10 Pa is preferable and at least 100 Pa is more preferable.
The mass spectrometry apparatus preferably feeds the gas from the second gas source also to a region where the metal ions fly until striking the reaction region and sets the pressure of the region to at least 10 Pa.
The mass spectrometry apparatus preferably does not provide the reaction chamber as a separate part but configures it using as common regions a region of placement of the emitter, the flight region, and a reaction region corresponding to the reaction chamber. Since the reaction chamber can be eliminated, the configuration becomes simpler and the manufacturing cost can be reduced.
In the mass spectrometry apparatus, preferably the halide compound Includes a carbon atom and a fluorine atom.
In the mass spectrometry apparatus, preferably the metal ion is any of Li, K, Na, Rb, Cs, Al, Ga, and In.